The present invention generally relates to an apparatus for measuring total gas in a fluid flowing in a process line.
Entrained gases are gases that exist in a gaseous form, mixed in the process fluid. For many industrial applications with small, less than ˜20% gas fraction by volume, the gas is typically in the form of small bubbles contained in a liquid continuous mixture. Entrained gases exist as either free bubbles moving within the stock or as bound (or residual) air that is adhered to the fiber. In either cases, entrained gas can generally be detected by monitoring the compressibility of the mixture and correlating the compressibility to volumetric percentage of entrained gas.
Dissolved gases are dissolved within the mixture on a molecular level. While in the solution, dissolved gases pose few operation problems. Typically dissolved gases have a negligible effect on the compressibility of the mixture. Thus, dissolved gases are difficult to detect via compressibility measurements. The sum of the entrained gases and the dissolved gases is defined as the total gases contained with a process mixture.
Monitoring levels of entrained and dissolved gases (e.g., air) is desirable in many industrial processes. For example, entrained and dissolve gases in the approach system of paper making machines are often problematic, leading to a wide variety of problems, including flow line pulsations, pin-holes in the produced paper, reduced paper sheet strength, and excessive build-up of aerobic growths.
Although dissolved gases are typically not problematic while dissolved, problems arise when dissolved gases come out of a solution as a result of either decreases in pressure or increases in temperature. One example of this is in pressurized head boxes on paper machines where the pressure drop associated with spraying the pulp/water mixture on to the paper machine can cause dissolved gases to come out of the solution and form entrained gas.
Various technologies exist to monitor dissolved gases in a process line. Typically, such technologies require that a representative sample of the process fluid be expanded to atmospheric conditions to liberate the dissolved air in the fluid, and the resulting entrained air is measured either directly, as in entrained gas testers (EGTs), by weight of the de-aerated fluid, as in so-called “bird bath” arrangements, or by ultrasonic measurement. While such technologies work well for some applications, the accuracy of these technologies may be sensitive to the velocity of the fluid. More specifically, as the velocity of the fluid sample is reduced, a situation is created wherein gas velocity is appreciably faster than liquid velocity (gas/liquid slip), resulting in a discrepancy between the actual dissolved air and the measured dissolved air. Thus, there remains a need for an accurate method of measuring dissolved and total air in a process fluid.